Recently, as effective uses of water resources, especially recycling uses thereof have been emphasized, purification treatments of wastewater and sewage water have been proposed. Among those, purification methods have been known in which water containing an organic compound is subjected to anaerobic and aerobic biological treatments for decomposing and removing the organic compound.
For example, PCT International Publication No. WO 03/106351 A1 proposes the production of a highly purified water by subjecting a large amount of by-product water by-produced in the Fischer-Tropsch process to a purification process including a biological treatment using anaerobic microorganisms and/or aerobic microorganisms. However, such by-product water synthesized in the Fischer-Tropsch process contains nitrogen and phosphorus components, which serve as nutrients for anaerobic microorganisms and aerobic microorganisms, either not at all or in amounts insufficient for the biological treatments. Accordingly, it is necessary to constantly add compounds containing nitrogen and phosphorus components. Likewise, plant wastewater discharged from chemical plants, petroleum plants, and petrochemical plants adopting processes other than the Fischer-Tropsch process contains nitrogen and phosphorus components either in amounts insufficient for the biological treatments or not at all. For this reason, when such a plant wastewater is subjected to biological treatments, it is necessary to constantly add compounds containing nitrogen and phosphorus components as nutrients for microorganisms for the biological treatments. Addition of the compounds containing nitrogen and phosphorus components as described above has been a cause of increased treatment costs for anaerobic and aerobic biological treatments of a plant wastewater.
It could therefore be helpful to provide a method of treating a plant wastewater and a treatment system therefor which allow treatment costs to be reduced for anaerobic and aerobic biological treatments on a plant wastewater containing a nitrogen component and/or a phosphorus component either in amounts insufficient for the biological treatments or not at all.